It is commonplace in the packaging of a wide variety of materials ranging from pharmaceutical products to instant coffee that a closure is provided in the form of a seal connected to the neck of a container and a screw cap covering and protecting the seal which provides a re-closable cap after the seal has been removed to gain access to the container. Often the closure is designed such that the underside of the seal has a heat sensitive adhesive coating or a meltable plastics layer covered by a metal foil. The seal is placed against the neck of a container and sandwiched against it by the applied screw cap. Upon induction heating, the metal foil is heated which activates the heat sensitive adhesive layer or melts the plastic layer so that on cooling, the seal bonds to the neck of the container.
A problem encountered with such seals is a difficulty in removing the seals from containers. In this regard, this has been overcome by including a tab extending sideways from the neck of the container which may be gripped by the consumer to facilitate removal of the seal. An example of such a system is the so called “Top Tab” structure which is described fully in U.S. Pat. No. 4,961,986. This system includes a multilayer substrate which is partly delaminated to provide a tab lying usually within the circumference of the container neck. In U.S. Pat. No. 4,961,986 this is achieved by forming the substrate from multiple layers which are adhered together across only a part of their extent. U.S. Pat. No. 4,961,986 further describes that the screw cap may include some form of liner in addition to the seal material. In such a two component system, wherein the seal and liner are provided separately, is that the two components have to be fitted in two separate operations.
As this is expensive and increases the complexity of the fitting process, there has been a focus on the development of a one component seal and liner system which avoids the need for two separate fitting operations. For example, EP-A-1472153 describes a one component seal and liner system, for attaching into a screw cap, which includes a tab. In the product detailed, the seal portion of the system is adhered to the liner portion by means of a release layer such that the seal and liner release from each other with a peel strength in the range from 20 to 90 g at a rate of 1500 mm/min on a sample strip 25 mm wide.
A further tab system is the successful “Lift “n” Peel”® commercial system. This can be comprised of a primary seal and a secondary liner or just a primary seal and is usually just a primary seal. The tab is formed by interposing a layer of polyethylene terephthalate into the primary seal which extends only over a portion of the surface area of the liner. The layer of polyethylene terephthalate is interposed between the foam layer and top EVA/PET layer. On heating, the EVA adheres to the interposed layer, and portion of foam still exposed and thus by virtue of the interposed plastics layer a tab, lying wholly within the boundaries of the liner, is formed.
In WO 97/02997, a method for including a tab in a primary seal laminate is disclosed. In this case three feeds are passed to a chill roll in contact with a nip roll. The first feed which will form the top layer of the primary seal laminate, is an ethylene-vinyl alcohol barrier layer sandwiched between two polypropylene layers. The second feed is a deadening member which may be PET and the third feed is extruded tabstock e.g. polypropylene, which forms a layer across the entire width of the laminate forming the top layer. All three feeds come into contact at the point where the chill roll and nip roll are in contact. The chill roll quenches the extruded tabstock maximising its amorphous properties and causes lamination of all three layers.
A problem which can be identified with such systems is that in attaching the system including the tab to a container to be sealed, an uneven level of bonding to the container is achieved with there being a propensity for stronger bonds to be formed under the tabbed portion of the liner as compared to the non-tabbed portion. There is a further danger that on heating a metal foil of an induction system, the top layer of the seal may burn where the heat transferred to this layer is too great.
A further problem encountered with such seals is that when used on containers which are to be stored at low temperatures, for example, in dairy applications, after storage at such temperatures, the bond between the seal substrate and tab has a tendency to fail. This means that when the seal is removed from the container on opening, because the bond fails, the tab comes away from the seal substrate leaving the seal substrate adhered to the container rather than removing the seal substrate from the container.
It is therefore clear that there is a need for a simple and cost effective method for including a tabstock in a primary seal laminate which solves the problems of requiring producing an effective tab and achieving an even level of bonding to the container to be sealed.